The invention relates to a method for determining the cooling action of a flowing gas atmosphere on workpieces, especially in the hardening of workpieces of steel, by a measuring body which is provided with at least one temperature sensor and heated to workpiece temperature and is exposed to the gas atmosphere.
It is known through the xe2x80x9cEnzyklopxc3xa4die Naturwissenschaften und Technik,xe2x80x9d 1981, Zweiburgen Verlag Weinham, Vol. E-J, key word xe2x80x9cHitzdrahtxe2x80x9d on page 1851, to determine the velocities of gases by blowing them against an electrically heated resistance with a temperature-related characteristic. This heated resistance has a length of about 1 nm and a thickness of a few xcexcm and virtually no time lag. In the event of a lowering of temperature by cooling, the original temperature and the original resistance are restored by increasing the current by means of a complex control system. For the determination of quenching curves of workpieces in metallurgical processes such an extremely delicate hot wire anemometer is neither foreseen nor suitable.
For this purpose, the workpieces or workpiece batches are quenched to harden them in a quenching chamber within a given time to temperatures below the perlite, bainite and/or martensite temperatures depending on the particular workpiece. The quenching chamber is designed for pressures up to 50 bar and in some cases higher, and hydrogen, helium, nitrogen or mixtures of at least two of these gases are used preferentially as quenching gases. These gases are fed through the batch(s) and removed again by a circulation blower not represented. On their way the quenching gases are passed through a heat exchanger, not shown, and recooled.
The driving power required for the gas circulation increases with pressure but decreases with the atomic weight of the quenching gases, so that hydrogen and helium gases or mixtures thereof are to be given preference, inasmuch as also the transfer of heat to these gases is especially good and the quenching rate is increased. In this case the transfer of heat to the workpieces but also to the heat exchangers is important.
EP 0 313 888 B2 describes first heating workpieces of steel, especially low-alloy steels that are difficult to harden, and/or workpieces of large or complex shape, and then quenching and hardening them with gases from the group, helium, hydrogen and nitrogen, and by gaseous mixtures of at least two gases of this group, at pressures between 10 and 40 bar. This is intended to eliminate the classical hardening methods using water, oils and salt baths with their adverse effect on the environment. The hardening is performed by means of these gases, which are circulated by means of a blower at a high velocity within the apparatus through a heat exchanger and the workpieces or batches of workpieces. The hardening can be performed in a heated single-chamber furnace or in an attached special quenching chamber belonging to the furnace. In the said disclosure the background for the elimination of the known hardening methods is also given.
In such quenching methods the procedure has formerly been to provide parts of a stationary batch with thermocouples. When this was not possible, so-called passive alpha probes have been added to the batch, i.e., special probes provided with thermocouples without a heating device, which are heated by thermal transfer from the adjacent workpieces. The quenching of the workpieces or components was determined by the measurements obtained (offprint xe2x80x9cIpsen Reportxe2x80x9d of the Ipsen company, article by B. Edenhofer, xe2x80x9cSteuerung der Hochdruckgasabschreckung mittels Wxc3xa4rmestromsensorxe2x80x9d [control of high-pressure gas quenching by means of a heat flow sensor] of October 1995). In this case measurements of previously run batches are used as a guide for fresh batches.
Such methods of measurement, however, are not possible in the case of moving batches in continuously operating apparatus with so-called xe2x80x9ccold chambers,xe2x80x9d since the batches are passed through individual chambers of the apparatus and the individual chambers are separated from one another by pressure-tight slides. Therefore, in such apparatus the control of the quenching action is performed by monitoring xe2x80x9csecondary factorsxe2x80x9d such as gas pressure, gas temperature, cooling water temperature, as well as the power input of the blower motors for circulating the gas. The determination of the quenching rate from these factors, however, is possible only with a great deal of mathematic calculation, and even then it is highly inaccurate due to measurement tolerances. Such indirect measurements and calculations therefore do not satisfy quality assurance requirements in modem manufacturing processes.
The invention is therefore addressed to the problem of providing a method and an apparatus by which the cooling action and quenching effect, and the time and temperature factors can be determined continuously and directly even in the case of large batches, so that possible adjustments can be performed extremely fast, i.e., in fractions of a second. This is to bring it about that the cooling or quenching, and hardening if desired, of all workpieces of a batch can be performed very quickly according to their hardening specifications.
Particularly the heat transfer from the workpieces or batch of workpieces to the cooling gas is to be controlled in order to prevent harmful heat tensions and/or irregular product quality, and furthermore transfer from the cooling gas to the heat exchanger is to be controlled, because the processes occurring at the workpiece surfaces and at the surfaces of the heat exchanger have an effect on one another.
The solution of the stated problem is accomplished in the method cited in the beginning by the fact that the measuring body is disposed outside of the workpieces and heated by a heating device associated with it to a given starting temperature and is then exposed to the flowing gas atmosphere, and that the cooling time curves measured on the measuring body are measured.
The stated problem is solved to the full extent by the solution according to the invention, and especially the cooling action or quench effect and the temperature time curve are continuously and directly determined, even in large batches, so that any necessary adjustments can be performed extremely fast, i.e., in fractions of a second. Thus it is accomplished that all workpieces of a batch are cooled or quenched and in some cases hardened, with great speed, in a measured manner according to their hardening specifications.
At the same time, it is especially the thermal transfer from the workpieces or batch of workpieces to the cooling gas that becomes controllable, and any harmful distortions due to thermal tensions and/or irregular production properties are avoided, and furthermore, also the thermal transfer from the cooling gas to the heat exchangers can be controlled, because the processes at the workpiece surfaces and at the surfaces of the heat exchanger again influence one another. What is involved is to some extent a synergistic effect. The more difficult the workpieces are to harden, for example in the case of low-alloy pieces and hard-to-harden workpieces and workpieces of large dimensions and complex shapes, different wall thicknesses, etc., the more important the employment of the invention becomes.
Pursuant to additional embodiments of the invention it is especially advantageous if-either singly or in combination:
the cooling time curves are compared with set patterns and if the differences between the actual values and the set patterns are used to control at least one factor from the group: gas pressure, gas velocity and cooling performance of a heat exchanger,
if the workpieces are preheated in a heating chamber, the measuring body is heated to the given initial temperature before the workpieces are brought into a quenching chamber equipped with the measuring body, and if after the workpieces are brought into the quenching chamber the heating of the measuring body is interrupted and the measuring body is exposed to the gas atmosphere circulating in the quenching chamber,
the temperature of the gas atmosphere is measured with an additional heat sensing device independent of the measuring body and the thermal transfer coefficient is determined in consideration of the measurements made by the heat sensing device of the measuring body,
the heating of the measuring body is performed by an induction coil surrounding the measuring body and/or by a heating device (e.g., a heating cartridge) disposed in the measuring body as a heating device, and/or by the fact that the measuring body is heated by passing a current directly through it,
the temperature curve through a temperature sensor disposed in the surface area of the measuring body is determined, and/or if
the temperature curve is determined by a temperature sensor arranged in the center of the measuring body.
The invention also relates to an apparatus for determining the cooling action of a flowing gas atmosphere on workpieces, especially in the hardening of workpieces of steel, by means of a measuring body provided with at least one temperature sensor and heated to workpiece temperature, which is exposed to the gas atmosphere.
For the solution of the same problem, such an apparatus is constructed according to the invention so that with the measuring body disposed outside of the workpieces there is associated its own heating device with a current source by which the measuring body can be heated to a given initial temperature.
In further embodiments of the apparatus of the invention, it is especially advantageous if either individually or in combination:
the heating device associated with the measuring body is an induction coil surrounding the measuring body, a heating device arranged in the measuring body, or the measuring body itself, to which for this purpose a low-voltage current source is applied in the circuit,
to detect the temperature of the gas atmosphere an additional temperature sensor independent of the measuring body is provided by which the heat transfer coefficient can be determined allowing for the measurements made by the temperature sensor of the measuring body,
the temperature sensor of the measuring body is switched to a central unit with storage areas, in which the time curves of the measurements made by the temperature sensors can be compared with established and stored set curves,
the electric power source of the heating device can be shut off by a central unit after reaching the measuring body""s starting temperature which can be preset in the central unit,
the central unit is connected through a control line to a medium frequency generator to supply the induction coil, and the induction coil, when the measuring body reaches the starting temperature established in the central unit, can be shut off by the central unit,
the measuring body is adapted, in regard to at least one of the factors: material, mass, geometry and emissivity, to the corresponding factors of the workpieces,
the measuring body is in the form of cylinder, and/or, if
the measuring body (5) is formed from an austenitic alloy with a low emission coefficient.
The invention also relates to the application of the method according to claim 1 and the apparatus according to claim 10 to the high-pressure gas quenching of workpieces in a quenching chamber with a heat exchanger at gas pressures between 5 and 50 bar, preferably between 10 and 40 bar.